Spindle motor having recessed stator coils

ABSTRACT

A magnet is provided on the inner circumferential surface of a hub rotatable about a central axis C of the motor, and a stator is arranged inside the inner circumferential surface of the hub to oppose the magnet. The stator core of the stator is formed by stacking substantially annular, plate-shaped core forming members, and has an annular portion arranged to be coaxial with the central axis, a plurality of winding portions projecting from the annular portion radially, and pole portions formed on the extending ends of the respective winding portions and opposing the magnet. Each winding portion is squeezed in the stacking direction by plastic formation such that its thickness is smaller than those of the annular portion and pole portion. A coil is wound on each squeezed winding portion.

This is a divisional of application Ser. No. 08/866,976, filed Jun. 2,1997 U.S. Pat. No. 5,798,583.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a spindle motor, a method ofmanufacturing the spindle motor, and a magnetic disk drive having aspindle motor.

2. Description of the Related Art

In recent years, a so-called brushless DC servo motor is widely employedas a spindle motor used in a magnetic disk drive. In particular, in themagnetic disk drive, in order to decrease the size and increase thecapacity of the magnetic disk drive, a so-called in-hub type spindlemotor having a magnetic circuit portion in a hub that supports amagnetic disk is used to improve the space efficiency.

As such an in-hub type spindle motor, a motor having a double-sidedsupport structure and a motor having a cantilevered support structureare provided. With the spindle motor having a double-sided supportstructure, a hub serving as a rotor is rotatably supported through abearing by a stationary support shaft provided upright on the bottomwall of the magnetic disk drive, and the upper end of the support shaftis connected to the cover of the magnetic disk drive, thereby improvingthe rigidity of the stationary portions including the support shaft.

In the spindle motor having a cantilevered support structure, a bearingis provided in a cylindrical support projecting from the bottom wall ofa magnetic disk drive, and a shaft integrally formed with the centralportion of a hub that serves as a rotor is rotatably supported by thebearing.

Usually, the magnetic circuit portion of the spindle motor of thesetypes is constituted by a back yoke, a magnet, a stator core, and coils,and is disposed between the inner circumferential surface of the hub andthe bearing. The back yoke is integrally formed with the circumferentialwall of the hub, and the magnet is formed annularly and is fixed to theinner circumferential surface of the hub.

Regarding the stator core, for example, a silicon steel plate having athickness of 0.2 to 0.4 mm is press-cut to form a core member having apredetermined shape. A plurality of core members each obtained in thismanner are stacked, thereby forming the stator core. The stator core hasan inner cylindrical portion, a plurality of winding portions extendingradially from the outer surface of the cylindrical portion, and wide endportions located at the extending ends of the respective windingportions. Slits are formed between adjacent winding portions. A coil iswound on the outer surface of each winding portion.

The upper and lower end portions of the coil wound on each windingportion project from the upper and lower surfaces, respectively, of thestator core. Hence, the effective thickness of the magnetic circuitportion becomes a sum of the projecting heights of the upper and lowerend portions of the coil to the thickness of the stator core in thestacking direction. When designing a spindle motor, the stacking heightof the stator core must be designed by considering the projectingheights of the end portions of the coils. In particular, when decreasingthe size of the magnetic disk drive, the stacking thickness or height ofthe stator core is limited within a predetermined range as it largelyinfluences the thickness of the entire spindle motor and the thicknessof the magnetic disk drive.

To increase the capacity of the magnetic disk drive, the number ofmounted magnetic disks must be increased. As the number of mountedmagnetic disks is increased, the magnetic circuit efficiency, inparticular the torque characteristics, of the spindle motor must beimproved. Various methods are possible to improve the torquecharacteristics of the spindle motor, i.e., the torque characteristicsof the magnetic circuit portion. Among these methods, one effectivemethod is to increase the stacking thickness of the stator core, therebyincreasing the motor constant.

More specifically, the motor constant is described by (torqueconstant)/(coil resistance)^(1/2), and is proportional to the 1/2th or3/4th power of the stacking thickness of the stator core depending onthe width and stacking thickness of the stator core. Accordingly, toincrease the stacking thickness of the stator core is effective inincreasing the motor constant.

As described above, however, the space in the magnetic disk drive wherethe spindle motor can be set is limited, and the stacking thickness ofthe stator core is limited accordingly. Further, the heights of the endportions of the coil wound on each winding portion of the stator coreare added to the effective stacking thickness of the stator core, makingit further difficult to increase the stacking thickness of the statorcore.

In particular, in a low-profile spindle motor, i.e., in a spindle motorhaving a stator core with a small stacking thickness, the projectingheights of the end portions of the coil occupy a large proportion in theheight of the entire magnetic circuit portion. This serves as a majorobstacle in increasing the stacking thickness of the stator core.

Due to the above reasons, the conventional spindle motor has variedproblems when improving its torque characteristics, and it is difficultto improve the torque characteristics without increasing the entirethickness.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above problems, andhas as its object to provide a spindle motor capable of improving itsmagnetic circuit characteristics without increasing its entirethickness, a method of manufacturing the spindle motor, and a magneticdisk drive having this spindle motor.

In order to achieve the above object, according to the presentinvention, there is provided a spindle motor comprising a rotorsupported to be rotatable about a central axis and having an innercircumferential surface provided with a magnet, and a stator providedinside the inner circumferential surface to oppose the magnet. Thestator core of the stator has an annular portion arranged to be coaxialwith the central axis, a plurality of winding portions projecting fromthe annular portion radially with respect to the central axis, and poleportions formed on the extending ends of the winding portions andopposing the magnet.

Each winding portion is squeezed in the direction of the central axis byplastic formation such that its thickness in the direction of thecentral axis is smaller than those of the annular portion and poleportion. A coil is wound on each squeezed winding portion.

In the spindle motor having the above arrangement, each winding portionof the stator core is squeezed to have a thickness smaller than those ofthe annular portion and pole portion. Thus, when a coil is wound on eachwinding portion, the coil end portions in the axial direction of thestator core are entirely or at least partly embedded in the stator core,and the projecting heights of the coil end portions from the upper andlower surfaces of the stator core are greatly reduced. Therefore, thethickness of the stator core itself in the axial direction can beincreased without increasing the entire height of the magnetic circuitportion including the stator and the coil. Because of the increase inthickness of the stator core, the magnetic circuit characteristics areimproved without increasing the thickness of the entire spindle motor.

To set the same magnetic circuit characteristics as in the conventionalspindle motor, the winding portions of the stator core are squeezed todecrease the projecting amounts of the coil end portions that projectfrom the stator core, so that the thickness of the entire stator isdecreased, thereby decreasing the size of the spindle motor.

According to the present invention, there is also provided a method ofmanufacturing a spindle motor including a rotor supported to berotatable about a central axis and having an inner circumferentialsurface provided with a magnet, and a stator arranged inside the innercircumferential surface to oppose the magnet, the method comprising thesteps of:

forming a substantially annular stator core having an annular portion, aplurality of winding portions extending from the annular portionradially outward with respect to the central axis, and pole portionsformed on extending ends of the winding portions and opposing themagnet; squeezing each of the winding portions in a direction of thecentral axis by plastic formation such that a thickness thereof in thedirection of the central axis is smaller than those of the annularportion and each of the pole portions; and winding a coil on eachsqueezed winding portion, thus constituting a stator.

When a stator core is formed by stacking a plurality of core formingmembers, after the core forming members are stacked, each windingportion is squeezed to be thin, and thereafter a coil is wound on eachwinding portion, thereby forming a stator.

According to the method of manufacturing the spindle motor, since thewinding portions are squeezed in advance of winding coils on them,deformation in stator core during plastic formation is prevented to formwinding portions having a small thickness. Deformation in the statorcore during plastic formation can be prevented easily by, e.g.,squeezing the winding portions while the inner and outer circumferentialsides of the stator core are respectively held with jigs.

Furthermore, according to the present invention, there is provided amagnetic disk drive comprising a magnetic disk, a spindle motor forrotating the magnetic disk, and recording/reproducing means forrecording/reproducing information on/from the magnetic disk. The spindlemotor includes a rotor arranged to be rotatable about a central axis,having an inner circumferential surface on which a magnet is provided,and supporting the magnetic disk, and a stator arranged inside the innercircumferential surface of the rotor to oppose the magnet.

The stator core of the stator has an annular portion arranged to becoaxial with the central axis, a plurality of winding portionsprojecting from the annular portion radially with respect to the centralaxis, and pole portions formed on extending ends of the respectivewinding portions and opposing the magnet. Each winding portion issqueezed in the direction of the central axis by plastic formation suchthat its thickness in the direction of the central axis is smaller thanthose of the annular portion and pole portion. A coil is wound on eachsqueezed winding portion.

In the magnetic disk drive having the above mentioned arrangement, eachwinding portion of the stator core of the spindle motor is squeezed tohave a thickness smaller than those of the annular portion and poleportion. When coils are wound on these winding portions, the projectingamounts of the end portions of the coil in the axial direction of thestator core are decreased. Then, the thickness of the stator core in theaxial direction can be increased without increasing the entire height ofthe stator including the coils. Due to the increase in thickness of thestator core, the magnetic circuit characteristics can be improvedwithout increasing the thickness of the entire spindle motor.

Since a spindle motor having improved magnetic circuit characteristicsis realized, an increase in starting speed of the magnetic disk drive,an increase in stability against the load, and an increase in capacityin accordance with an increase in the number of mounted magnetic diskscan be attained.

To set the same magnetic circuit characteristics as in a conventionalspindle motor, the winding portions of the stator core are squeezed todecrease the projecting amounts of the end portions of the coils woundon these winding portions, so that the thickness of the entire stator isdecreased, thus decreasing the size of the spindle motor. Then, theentire magnetic disk drive can be formed to have a low profile.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed out in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate presently preferred embodiments ofthe invention and, together with the general description given above andthe detailed description of the preferred embodiments given below, serveto explain the principles of the invention.

FIGS. 1 to 7 show a magnetic disk drive according to a first embodimentof the present invention, in which:

FIG. 1 is an exploded perspective view of the magnetic disk drive,

FIG. 2 is an enlarged sectional view showing a spindle motor of themagnetic disk drive,

FIG. 3 is a plan view schematically showing a magnetic circuit portionof the spindle motor,

FIG. 4 is an enlarged perspective view showing part of the stator of thespindle motor,

FIG. 5 is a perspective view showing part of a stator core before itswinding portion is subjected to pressing,

FIG. 6 is a perspective view showing part of the stator core after itswinding portion is subjected to pressing, and

FIGS. 7A to 7H are side views schematically showing the manufacturingsteps of the stator and conventional stators;

FIG. 8 is a sectional view showing a jig used in pressing the statorcore;

FIG. 9 is a perspective view showing an essential part of a stator coreof a spindle motor according to a second embodiment of the presentinvention;

FIG. 10 is a perspective view showing essential parts of core formingmembers used in a spindle motor according to a third embodiment of thepresent invention;

FIG. 11 is a perspective view showing a stator core obtained by stackingcore forming members shown in FIG. 10;

FIG. 12 is a sectional view taken along line XII--XII of FIG. 11;

FIG. 13A is a perspective view showing an essential part of a first coreforming member used in a spindle motor according to a fourth embodimentof the present invention;

FIG. 13B is a perspective view showing an essential part of a secondcore forming member used in the spindle motor according to the fourthembodiment of the present invention;

FIG. 14 is a perspective view showing a stator core formed by stackingthe first and second core forming members;

FIG. 15 is a sectional view taken along line XV--XV of FIG. 14;

FIG. 16A is a perspective view showing an essential part of a first coreforming member used in a spindle motor according to a fifth embodimentof the present invention;

FIG. 16B is a sectional view taken along line XVI--XVI of FIG. 16A;

FIG. 16C is a perspective view showing an essential part of a secondcore forming member used in the spindle motor according to the fifthembodiment of the present invention;

FIG. 17 is a partially sectional side view showing the arrangement ofthe first and second core forming members of the fifth embodiment;

FIG. 18 is a partially sectional side view of a stator core formed bystacking the first and second core forming members of the fifthembodiment; and

FIG. 19 is an enlarged sectional view of a spindle motor portion of amagnetic disk drive comprising a high-profile spindle motor according toa sixth embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Magnetic disk drives having spindle motors according to the preferredembodiments of the present invention will be described with reference tothe accompanying drawings.

As shown in FIG. 1, a magnetic disk drive has a housing 10. The housing10 has a main body 12 formed in a rectangular box-like shape with anupper end opening. More specifically, the main body 12 includes the mainbody having a rectangular bottom wall and side walls provided upright onthe side edges of the bottom wall, and a top cover 14 fixed to the mainbody 12 with a plurality of screws 11 to close the upper end opening ofthe main body 12. A packing 13 is interposed between the top cover 14and the main body 12.

A magnetic disk 16, a rotating mechanism 18, a pair of magnetic headassemblies 20, a carriage assembly 22, a voice coil motor 24, apreamplifier 21, and the like are housed in the housing 10. The magneticdisk 16 serves as a magnetic recording medium. The rotating mechanism 18supports and rotates the magnetic disk 16. Each of the pair of magnetichead assemblies 20 has a magnetic head that serves asrecording/reproducing means for recording/reproducing informationon/from the magnetic disk 16. The carriage assembly 22 pivotallysupports these magnetic head assemblies 20. The voice coil motor 24rotates and positions the carriage assembly 22. A printed circuit board(not shown) for controlling the operations of the rotating mechanism 18,the voice coil motor 24, and the magnetic head assemblies 20 is fixed tothe outer surface of the main body 12 with screws, to oppose the bottomwall of the main body 12.

As shown in FIG. 2, the rotating mechanism 18 has a spindle motor 25using a brushless DC servo motor. The spindle motor 25 has a bracket 27which is fitted in a through hole 26 formed in the bottom wall 12a ofthe housing 10 and fixed to the bottom wall 12a with a plurality ofscrews. A cylindrical support 28 is integrally formed with the centralportion of the bracket 27 to project upward from the bracket 27. A hub30 serving as the rotor of the spindle motor 25, and a stator 32 aresupported by the support 28.

More specifically, the hub 30 integrally has a cylindricalcircumferential wall 34 and a shaft portion 35 coaxially provided insidethe circumferential wall 34. The shaft portion 35 is inserted in thesupport 28 and supported by it through a pair of ball bearings 36.Accordingly, the hub 30 is supported by the support 28 to be rotatableabout a central axis C. The circumferential wall 34 has an annularflange 37 projecting outward in the radial direction. An annular magnet38 forming part of a magnetic circuit (to be described later) is fixedto the inner circumferential surface of the circumferential wall 34. Thehub 30 is made of a magnetic material, and the circumferential wall 34has a function of a back yoke.

The magnetic disk 16 has an inner hole at its central portion, and isplaced on the bracket 27 such that the upper end portion of the hub 30is fitted in this inner hole. A disk-shaped clamper 39 consisting of aplate spring is mounted on the upper end of the hub 30 with a screw 40,and the inner peripheral portion of the magnetic disk 16 is clampedbetween the peripheral edge portion of the clamper 39 and the flange 37.Thus, the magnetic disk 16 is held on the hub 30 and is rotatabletogether with the hub 30 about the central axis C.

The stator 32 of the spindle motor 25 has a stator core 42 fixed to theouter circumferential surface of the support 28 to be located inside thecircumferential wall 34 of the hub 30, and a plurality of coils 46mounted on the stator core 42.

The magnetic circuit of the spindle motor 25 will be described in detailtogether with the arrangement of the stator 32. As shown in FIGS. 2 to4, the annular magnet 38 is magnetized to have a number of poles, i.e.,12 poles in this embodiment.

The stator core 42 has an annular or cylindrical portion 43, ninewinding portions 44, and pole portions 45. The annular portion 43 isfitted on the outer circumferential surface of the support 28. The ninewinding portions 44 extend from the outer circumferential surface of theannular portion 43 outward in the radial direction with respect to thecentral axis C. The pole portions 45 are formed on the extending ends ofthe respective winding portions 44. The winding portions 44 are providedequidistantly in the circumferential direction of the annular portion43. A coil 46 is wound on each winding portion 44. Each pole portion 45is formed to be larger than the width of the corresponding windingportion 44 in the circumferential direction of the annular portion 43,and opposes the inner circumferential surface of the magnet 38 with asmall gap.

Each coil 46 is electrically connected to the circuit board (describedabove) through a flexible circuit board 47. Hence, when the coils 46 areenergized, a magnetic flux Φ generated by each coil 46 flows through thecorresponding winding portion 44, the corresponding pole portion 45, themagnet 38, the adjacent pole portion 45, the adjacent winding portion44, and the annular portion 43. Therefore, the hub 30 is driven by themutual operations of the formed magnetic fields to rotate about thecentral axis C.

The arrangement and the manufacturing method of the stator core 42 willbe described in further detail. In this embodiment, as shown in FIGS. 5and 6, the stator core 42 is formed by stacking a plurality of, e.g.,six, core forming members.

A core forming member 50 is formed substantially annularly bypress-cutting a silicon steel plate having a thickness of about 0.2 to0.4 mm with a pressing machine. Each core forming member 50 has anannular portion 43a, winding forming portions 44a, and pole portionforming portions 45a for forming the annular portion 43, the windingportions 44, and the pole portions 45, respectively, of the stator core42, and is formed to have the same planar shape as that of the statorcore 42. The six core forming members 50 have the same shape and samesize.

These core forming members 50 are stacked coaxially such that theirwinding forming portions 44a and the pole portion forming portions 45positionally coincide with one another, and are fixed to one another bycaulking. As shown in FIG. 5, when the core forming members 50 arestacked, the annular portion 43, the winding portions 44, and the poleportions 45 are located on the same plane, and each winding portion 44has a width W1 in the circumferential direction of the annular portion43.

Subsequently, the winding portions 44 of the stator core 42 formed bystacking are squeezed by pressing in the axial direction of the annularportion 43, i.e., from above and below in the stacking direction. Thethickness of each winding portion 44 in the stacking direction becomessmaller than the thickness of the annular portion 43 and the poleportion 45. Upon being squeezed, each winding portion 44 spreads to itstwo sides in the circumferential direction of the annular portion 43,and its width W2 becomes larger than the width W1 before pressing.

When squeezing the winding portions 44 of the stator core 42 bypressing, countermeasures must be taken so that the inner and outerdiameters of the stator core 42 and the shapes of the pole portions 45do not change due to the stress generated upon pressing. For thispurpose, in this embodiment, pressing is performed by holding the statorcore 42 with a jig, as shown in FIG. 8.

This jig has an inner jig 53 and an outer jig 55. The inner jig 53 isformed into a circular column having a diameter almost the same as theinner diameter of the annular portion 43, and is fitted in the innerhole of the annular portion 43. The outer jig 55 has a cylindricalportion 57 and a plurality of projecting portions 59. The cylindricalportion 57 has an inner diameter almost the same as the outer diameterof the stator core 42 and is fitted on the outer circumferential surfaceof the stator core 42. The projecting portions 59 extend from the innercircumferential surface of the cylindrical portion 57 toward the centerand can respectively enter slits among the winding portions 44 of thestator core 42.

These projecting portions 59 are formed such that a circle defined bytheir distal end portions has a diameter nearly equal to the outerdiameter of the annular portion 43 of the stator core 42. Eachprojecting portion 59 has a width smaller than that of the correspondingslit of the stator core 42. Thus, each projecting portion 59 is insertedin the corresponding slit such that its distal end face abuts againstthe outer circumferential surface of the annular portion 43 of thestator core 42, while maintaining a predetermined gap g with respect tothe winding portions 44 located on its two sides.

When the winding portions 44 are subjected to pressing while the innerjig 53 and the outer jig 55 having the above arrangement are mounted onthe stator core 42, the variations of the inner diameter of the annularportion 43 are prevented by the inner jig 53, and the variations of theouter diameter of the annular portion 43 are prevented by the projectingportions 59 of the outer jig 55. Any variations of the outer diameter ofthe stator core 42, including the deformation of the pole portions 45,are prevented by the cylindrical portion 57 and the projecting portions59 of the outer jig 55.

Since the gap g is formed between the winding portions 44 and theprojecting portions 59 of the outer jig 55, during pressing, the windingportions 44 can spread in the circumferential direction of the annularportion 43 by a distance corresponding to the gap g.

After the above pressing operation is performed, the coils 46 are woundon the corresponding squeezed winding portions 44, thereby forming thestator 32 shown in FIG. 4. After winding the coils 46, the stator 32 issubjected to a surface treatment with an epoxy-based resin by coating,electrodeposition, or the like, so that the insulation resistancebetween the coils 46 and the stator core 42 is ensured.

FIGS. 7A to 7C, and 7E to 7G schematically show the manufacturing stepsof the stator 32 described above. As shown in FIG. 7A, assuming that thestacking thickness of the stator core 42 before pressing is defined asS1, the winding portions 44 of the stator core 42 are squeezed untiltheir stacking thickness S2 reaches a predetermined value smaller thanthe stacking thickness S1 (S2<S1), as shown in FIG. 7B.

Subsequently, the coil 46 is wound on each winding portion 44. As shownin FIG. 7C, assuming that the height or thickness of each end portion ofeach coil 46 is defined as L, when the stacking thickness S2 of thesqueezed winding portion 44 is set to (S1-2L), a thickness S of theentire magnetic circuit portion including the magnet 38 and the stator32 after coil winding substantially coincides with the stackingthickness S1 of the annular portion 43 or the pole portion 45 of thestator core 42.

FIG. 7D shows a conventional magnet 38' and stator 32' which is formedby winding coils 46' on winding portions 44' of a stator core 42' formedby stacking four core forming members. Assuming that the stackingthickness of the stator core 42' is defined as S2 and that the thicknessof each end portion of the coil 46' is defined as L, the thickness ofthe entire stator 32' is S.

As is apparent from comparison with the conventional stator 32' shown inFIG. 7D, in the stator 32 of this embodiment shown in FIG. 7C, when thewinding portion 44 of the stator core 42 is formed thin by squeezing,the thickness L of the coil 46 wound on the winding portion 44 isabsorbed entirely or at least partly. Despite that the number of stackedcore forming members is increased from four to six, the thickness S ofthe entire stator 32 is maintained at substantially the same value asthat of the conventional stator 32'.

In the magnetic disk apparatus having the above arrangement, eachwinding portion 44 of the stator core 42 of the spindle motor 25 issqueezed in the stacking direction to be thinner than the stackingthickness of the annular portion 43 and the pole portion 45, and thecoil 46 is wound on each squeezed winding portion 44. Hence, thestacking thickness of the stator core 42 can be increased by increasingthe number of core forming members 50 stacked without increasing thethickness of the entire stator 32, i.e., the thickness of the spindlemotor 25 in the direction of the central axis C. Accordingly, the torquecharacteristics of the spindle motor 25 can be largely improved byincreasing the motor constant without changing the entire height of thespindle motor 25.

For example, when the number of core forming members stacked isincreased from four to six, as in the embodiment described above,assuming that the coil resistance is the same and the torque constant isproportional to the number of core forming members stacked, a motorconstant km is improved to 6/4=1.5 times.

Assume a spindle motor of a conventional arrangement in which the statoris constructed by winding coils having a coil end portion L=0.7 mm on astator core obtained by stacking 15 core forming members each having athickness of 0.3 mm. When a spindle motor having the same height as thatof the above spindle motor is formed with the arrangement of the aboveembodiment, the winding portions of the stator core are squeezed by anamount to absorb the height 2 L of the upper and lower end portions ofthe coil, for example, so that the number of core forming membersstacked can be increased to 19 by 2L/0.3=about 4 without changing theentire height S of the stator.

In this case, assuming that the coil resistance is the same and that thetorque constant is proportional to the number of core forming membersstacked, the motor constant km is improved to 19/15=1.27 times. From15/19=0.79, the winding portions need only be squeezed to about 80% thestacking thickness of the stator core.

When the stator core has a large stacking thickness, it is possible toset-a large winding portion squeezing amount. When, however, thesqueezing amount is excessively large, precision management of thestator core itself becomes difficult, and swelling of the windingportions in the circumferential direction becomes excessively large,thus interfering with coil winding. For this reason, the squeezingamount of the winding portions must be appropriately selected asrequired. Note that the squeezing amount may well increase upon futuredevelopments of the press technique.

As each winding portion 44 spreads in the circumferential direction uponbeing squeezed, its sectional area remains substantially the same asthat obtained before squeezing. Hence, even if the winding portions aredeformed thin by squeezing, the effective sectional area through whichthe magnetic flux flows does not decrease, and a decrease in magneticcircuit efficiency is not caused.

In the magnetic disk apparatus having the spindle motor 25 of the abovearrangement, when the torque characteristics are improved, as describedabove, various operations and effects can be obtained, e.g., reductionin starting time of the magnetic disk, increase in stability of therotation of the magnetic disk against the load, realization of anincrease in capacity in accordance with an increase in number of mountedmagnetic disks, and the like.

In the above embodiment, the stacking thickness of the stator core isincreased without changing the height of the spindle motor. However, asshown in FIGS. 7E to 7H, when a stator core 42 is formed by stacking thesame number of, e.g., four, core forming members, as in the stator 32'of the conventional spindle motor, with the present invention, theheight S of the entire stator can be decreased to be smaller than theheight S' of the entire stator of the conventional spindle motor whilemaintaining the stacking thickness S1 of the stator core 42 at the samelevel as in the conventional case. Accordingly, the spindle motor can bemade thin if the torque characteristics of the spindle motor 25 are tobe held at the same level as in the conventional case. As a result, thethickness of the entire magnetic disk drive can be decreased.

In the embodiment described above, a spindle motor whose stator core isformed by stacking a plurality of core forming members is described.However, a stator core 42 may be formed of a single plate, as shown inFIG. 9. In this case as well, a stator is formed by squeezing windingportions 44 of the stator core 42 by pressing to be thinner than anannular portion 43 and a pole portion 45, and winding coils on thesqueezed winding portions, and operations and effects identical to thosedescribed above can be obtained. Furthermore, a stator core can also beformed by stacking a plurality of core forming members each having thesame structure as that of the stator core having squeezed windingportions 44 as shown in FIG. 9. The same operations and effects as thosedescribed above can be obtained with this arrangement as well.

FIGS. 10 to 12 show the arrangement and the manufacturing method of astator core 42 of a spindle motor according to still another embodimentof the present invention. In this embodiment, as shown in FIG. 10, eachcore forming member 50 has an annular portion 43a, winding formingportions 44a extending from the annular portion 43a radially outward,and pole portion forming portions 45a formed on the extending ends ofthe respective winding forming portions 44a, and is formed of a thinplate into an annular shape. Each of the pole portion forming portions45a has an arcuated shape and is coaxial with the annular portion 43a.Each of the pole portion forming portion 45a is arranged such that anaxis b passing the circumferentially middle position of the pole portionforming portion 45a is offset from a central axis a of the correspondingwinding forming portion 44a in the circumferential direction. Inparticular, the amount of offset between the central axes a and b is setto about half the width of the winding forming portion 44a in thecircumferential direction of the annular portion 43a.

To form a stator core 42, first, core forming members 50 are stackedcoaxially such that they alternately face up and down and their poleportion forming portions 45a are aligned with each other, and are fixedto each other by caulking. Then, the winding forming portions 44a ofevery two adjacent core forming members 50 are offset from each other inthe circumferential direction by a distance corresponding to theirwidth.

In this state, as shown in FIGS. 11 and 12, the winding portions 44 ofthe stator core 42 are squeezed by pressing from above and below in thestacking direction. Hence, when forming the stator core 42 by stacking,e.g., four core forming members 50, the winding forming portion 44a ofeach of the uppermost and lowermost core forming members is deformeduntil it is level with the other adjacent winding forming portion 44a.Therefore, each winding portion 44 formed by these winding formingportions 44a has a thickness in the stacking direction, which is smallerthan the stacking thickness of the annular portion 43 and the poleportion 45.

After this pressing, coils are wound on the respective thinly squeezedwinding portions 44, thereby forming the stator.

Also in this still other embodiment having the above arrangement, thestacking thickness of the stator core 42 can be increased by increasingthe number of core forming members 50 stacked without increasing thethickness of the entire stator, i.e., the thickness of the spindle motorin the direction of the central axis C. Accordingly, the torquecharacteristics of the spindle motor can be largely improved byincreasing the motor constant without changing the entire height of thespindle motor.

In still another embodiment of the present invention shown in FIGS. 13Ato 15, a stator core 42 of a spindle motor is formed by stacking twotypes of core forming members. More specifically, as shown in FIGS. 13Aand 13B, the stator core 42 is formed by using first core formingmembers 50a and second core forming members 50b.

The first and second core forming members 50a and 50b each have anannular portion 43a, a plurality of winding forming portions 44aextending outward from the annular portion 43a in the radial direction,and pole portion forming portions 45a provided at the extending ends ofthe winding forming portions 44a, and have the same shape and sizeexcept for its winding forming portions 44a.

Each winding forming portion 44a of each first core forming member 50ais constituted by a pair of extending portions 54. These extendingportions 54 extend between the annular portion 43a and the pole portionforming portion 45a to be parallel to each other, and are separated fromeach other by a predetermined distance d1 in the circumferentialdirection.

Each winding forming portion 44a of each second core forming member 50bis constituted by a single extending portion 56 extending between theannular portion 43a and the pole portion forming portion 45a. A width d2of the extending portion 56 in the circumferential direction of theannular portion 43a is substantially the same as the predetermineddistance d1.

To form the stator core 42 with the first and second core formingmembers 50a and 50b, first, a plurality of pairs of first and secondcore forming members 50a and 50b are prepared, are stacked coaxiallysuch that their pole portion forming portions 45a are aligned, and arefixed to each other by caulking. Hence, the extending portion 56 of eachwinding forming portion 44a of each second core forming member 50b islocated to oppose the gap between the pair of extending portions 54 ofthe corresponding winding forming portion 44a of the adjacent first coreforming member 50a.

In this state, as shown in FIGS. 14 and 15, winding portions 44 of thestator core 42 are squeezed by pressing from above and below in thestacking direction. Thus, when forming the stator core 42 by stacking,e.g., two pairs of first and second core forming members 50a and 50b,the extending portions 54 of the winding forming portion 44a of each ofthe uppermost and lowermost first core forming members 50a are deformeduntil they are level with the extending portion 56 forming thecorresponding winding forming portion 44a of the adjacent second coreforming member 50b, and the extending portion 56 of the second coreforming member 50b is accommodated between these pair of extendingportions 54. Accordingly, the winding portions 44 of the stator core 42formed by these winding forming portions 44a have a thickness in thestacking direction, which is smaller than those of an annular portion 43and the pole portion 45.

After this pressing, coils are wound on the respective thinly squeezedwinding portions 44, thus forming a stator.

Also in this still other embodiment having the above arrangement, thestacking thickness of the stator core 42 can be increased by increasingthe number of core forming members stacked without increasing thethickness of the entire stator. Accordingly, the torque characteristicsof the spindle motor can be largely improved without changing the entireheight of the spindle motor. Note that the numbers of first and secondcore forming members stacked can be increased or decreased as required.

FIGS. 16A to 18 show still another embodiment in which a stator core isformed by stacking two types of core forming members in the same manneras in the above embodiment. More specifically, as shown in FIGS. 16A to16C, a stator core 42 is formed by using substantially annular first andsecond core forming members 50a and 50b each obtained by press-cutting athin plate.

The first and second core forming members 50a and 50b each have anannular portion 43a, a plurality of winding forming portions 44aextending outward from the annular portion 43a in the radial direction,and pole portion forming portions 45a provided at the extending ends ofthe winding forming portions 44a, and have the same shape and size. Eachwinding forming portion 44a of each first core forming member 50a isthin as it is squeezed in the direction of the thickness by pressing,and its two end portions in the circumferential direction of the annularportion 43a are plastic-deformed to spread in the circumferentialdirection. The two spread end portions are bent at substantially rightangles in the same direction to form a pair of bent portions 58.

A gap W1 between the bent portions 58 is set to be substantially equalto the width of the winding forming portion before plastic formation,i.e., substantially the same as a width W2 of each winding formingportion 44a of each second core forming member 50b.

As shown in FIGS. 17 and 18, to form the stator core 42 by using thefirst and second core forming members 50a and 50b, a plurality of, e.g.,two, second core forming members 50b are arranged between two first coreforming members 50a. The first and second core forming members 50a and50b are stacked coaxially such that their pole portion forming portions45a are aligned, and are fixed to each other by caulking. At this time,the upper and lower two first core forming members 50a are stacked suchthat bent portions 58 of their winding forming portions 44a projecttoward the second core forming members 50b.

Hence, the winding forming portion 44a of each second core formingmember 50b is fitted between the bent portions 58 of the correspondingwinding forming portion 44a of the adjacent first core forming member50a. Accordingly, the winding forming portions 44a of the two secondcore forming members 50b are surrounded by the winding forming portions44a of the first core forming members 50a, and these winding formingportions 44a form each winding portion 44 of the stator core 42. Thethickness of each winding portion 44 in the stacking direction issmaller than those of an annular portion 43 and the pole portion 45.

Also in this still other embodiment having the above arrangement, thestacking thickness of the stator core 42 can be increased by increasingthe number of core forming members stacked without increasing thethickness of the entire stator. Accordingly, the torque characteristicsof the spindle motor can be largely improved without changing the entireheight of the spindle motor. Since the first core forming members havingthe winding forming portions 44a that are squeezed in advance are used,the stator core after stacking need not be plastic-deformed, so that theprocessability of the spindle motor is improved while a sufficienteffective sectional area of the winding portion is maintained. The sameoperations and effects as those described above can be obtained even ifthe number of second core forming members stacked is increased asrequired.

In the plurality of embodiments described above, comparativelylow-profile spindle motors having six or less stacked core formingmembers are described. However, the present invention is not limited tothese, but can also be applied to a high-profile spindle motor having alarge number of stacked core forming members.

FIG. 19 shows a high-profile spindle motor 25 in the type of a brushlessDC servo motor and incorporated in a magnetic disk drive. This spindlemotor 25 has a bracket 27 which is fitted in a through hole 26 formed ina bottom wall 12a of a housing 10 of the magnetic disk drive and isfixed to the bottom wall 12a with a plurality of screws. A stationarysupport shaft 60 stands upright at the central portion of the bracket27. A hub 62 serving as the rotor of the spindle motor 25 is rotatablysupported by the support shaft 60.

More specifically, the hub 62 is formed to integrally have a cylindricalcircumferential wall 63 and a cylindrical support 64 coaxially providedinside the circumferential wall 63. The support 64 is arranged outsidethe support shaft 60 to be coaxial with it, and is supported by itthrough a pair of ball bearings 65. Accordingly, the hub 62 is supportedby the support shaft 60 to be rotatable about a central axis C.

The circumferential wall 63 has an annular flange 67 projecting from itsouter circumferential surface outward in the radial direction. Anannular magnet 66 forming part of a magnetic circuit is fixed to theinner circumferential surface of the circumferential wall 63. The hub 62is made of a magnetic material, and the circumferential wall 63 has afunction of a back yoke.

Three magnetic disks 16a, 16b, and 16c are mounted on the hub 62. Thesemagnetic disks are coaxially fitted on the outer circumferential surfaceof the circumferential wall 63 of the hub 62, and are stacked on theflange 67 in the axial direction. A pair of spacer rings 68 are fittedon the circumferential wall 63 of the hub 62 and located among each twoadjacent magnetic disks.

An annular fixing ring 70 is fixed to the upper end of the hub 62 with aplurality of screws 69 separated from each other in the circumferentialdirection. The fixing ring 70 has an outer diameter larger than theouter diameter of the hub 62 and the inner diameter of each magneticdisk 16. Hence, the magnetic disks 16a, 16b, and 16c are clamped betweenthe flange 67 formed on the hub 62 and the fixing ring 70 through thespacer rings 68, and are held on the hub 62 to be rotatable togetherwith the hub 62 about the central axis C.

A screw hole is formed in the upper end of the support shaft 60, and ascrew 72 is screwed in this screw hole through a through hole 71 formedin a top cover 14 of the magnetic disk drive. In other words, the lowerend of the shaft 60 is fixed to the bracket 27 of the spindle motor 25,and the upper end of the shaft 60 is fixed to the top cover 14, so thatthe support shaft 60 is supported with a double-sided support structure.

A stator 32 of the spindle motor 25 has a substantially cylindricalstator core 42 and a plurality of coils 46 mounted on the stator core42. The stator core 42 is fixed to the bracket 27, and is arrangedbetween the circumferential wall 63 and the support 64 of the hub 62 tobe coaxial with them.

In the same manner as in the embodiments described above, the statorcore 42 is formed by stacking a number of core forming members, and hasan annular portion 43, a plurality of winding portions 44, and poleportions 45 closely opposing a magnet 66. Each winding portion 44 issqueezed in the direction of the central axis C, i.e., in the stackingdirection, and its stacking thickness is smaller than the stackingthickness of the annular portion 43 and the pole portion 45. The coil 46is wound on each squeezed winding portion 44.

Also in this still other embodiment having the above arrangement, thestacking thickness of the stator core 42 can be increased by increasingthe number of core forming members stacked without increasing thethickness of the entire stator 32, i.e., without increasing thethickness of the spindle motor 25 in the direction of the central axisC. Accordingly, the torque characteristics of the spindle motor 25 canbe largely improved by increasing the motor constant without changingthe entire height of the spindle motor 25.

When the torque characteristics of the spindle motor 25 are improved,various operations and effects can be obtained, e.g., reduction instarting time of the magnetic disk, increase in stability of therotation of the magnetic disk against the load, realization of anincrease in capacity in accordance with an increase in number of mountedmagnetic disks, and the like.

The spindle motor and its manufacturing method according to the presentinvention are not limited to those of a magnetic disk drive, but can besimilarly applied to spindle motors used in other applications, as amatter of course.

As has been described above in detail, according to the presentinvention, a spindle motor, the magnetic circuit characteristics ofwhich are improved without increasing the entire thickness, and itsmanufacturing method can be provided, and a small-size spindle motorhaving a decreased entire thickness and its manufacturing method canalso be provided when the same magnetic circuit characteristics are set.

Furthermore, according to the present invention, a magnetic diskapparatus having the spindle motor described above and improved startingspeed, stability, and the like can be provided.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details, representative devices, andillustrated examples shown and described herein. Accordingly, variousmodifications may be made without departing from the spirit or scope ofthe general inventive concept as defined by the appended claims andtheir equivalents.

What is claimed is:
 1. A magnetic disk drive comprising:a magnetic disk; a spindle motor for rotating the magnetic disk; and recording/reproducing means for recording/reproducing information on/from the magnetic disk, said spindle motor including:a rotor supported to be rotatable about a central axis and having an inner circumferential surface on which a magnet is provided; and a stator arranged inside the inner circumferential surface to oppose the magnet, said stator including:a stator core formed by coaxially stacking a plurality of plate-shaped annular core forming members, each annular core forming member having an annular portion coaxial with the central axis, a plurality of winding portions extending from the annular portions radially with respect to the central axis, and pole portions formed on the extended ends of the respective winding portions and opposing the magnet, the entire plurality of winding portions of all stacked core forming members being squeezed in a direction of the central axis and having a thickness in the direction of the central axis, which is smaller than those of the annular portions and each of the pole portions, and a coil wound on the entire plurality of squeezed winding portions.
 2. A magnetic disk drive according to claim 1, wherein said stator core includes annular, plate-shaped first and second core forming members which are stacked coaxially,the first core forming member has an annular portion, a plurality of winding forming positions extending from the annular portion radially outward, and pole portion forming portions formed at extending ends of the respective winding forming portions, each of the winding forming portions being squeezed in a direction of plate thickness and having a pair of bent portions which are located at two end portions of the winding forming portion in a circumferential direction of the annular portion and bent in an axial direction of the annular portion, the second core forming member has an annular portion, a plurality of winding forming portions extending from the annular portion radially outward, and pole portion forming portions formed at extending ends of the respective winding forming portions, and the first and second core forming members are stacked such that at least one second core forming member is interposed between two first core forming members, and each winding forming portion of the second core forming member is located between two band portions of a corresponding winding forming portion of the first core forming member.
 3. A magnetic disk drive according to claim 1, wherein each of the coil winding portions is squeezed by forces applied thereto in opposite directions such that a squeezed extent decreases from the outermost core forming members in the coil winding portion to a central core forming member therein.
 4. A magnetic disk drive according to claim 3, wherein the squeezed portions of the core forming members in each of the winding portions are in tight contact with each other in the direction of the central axis such that there are no air gaps between corresponding winding portions of the core forming members, and wherein the coil wound on each of the squeezed winding portions is at least partly embedded in the stator core.
 5. A magnetic disk drive comprising:a magnetic disk; a spindle motor for rotating the magnetic disk; and recording/reproducing means for recording/reproducing information on/from the magnetic disk; said spindle motor including:a rotor supported to be rotatable about a central axis and having an inner circumferential surface on which a magnet is provided; and a stator arranged inside the inner circumferential surface to oppose the magnet; said stator including,a stator core having an annular portion arranged to be coaxial with the central axis, a plurality of winding portions projecting from the annular portion radially with respect to the central axis, and pole portions formed on extending ends of the respective winding portions and opposing the magnet, each of the winding portions being squeezed in a direction of the central axis and having a thickness in the direction of the central axis which is smaller than those of the annular portion and each of the pole portions, and a coil wound on each of the squeezed winding portions; said stator core having annular, plate-shaped first and second core forming members which are stacked alternately and coaxially, the first core forming member having an annular portion, a plurality of winding forming portions extending from the annular portion radially outward, and pole portion forming portions formed at extending ends of the respective winding forming portions, each of the winding forming portions having a pair of extending portions arranged with a predetermined gap in a circumferential direction of the annular portion, the second core forming member having an annular portion, a plurality of winding forming portions extending from the annular portion radially outward, and pole portion forming portions formed at extending ends of the respective winding forming portions, each of the winding forming portions of the second core forming member having a width substantially equal to the gap between the pair of extending portions of the first core forming member, and each of the winding portions of the stator core formed by stacking the first and second core forming members being squeezed in a stacking direction thereof, and each of the winding forming portions of the second core forming member being accommodated between the extending portions of a corresponding winding forming portion of the first core forming member.
 6. A magnetic disk drive comprising:a magnetic disk; a spindle motor for rotating the magnetic disk; and recording/reproducing means for recording/reproducing information on/from the magnetic disk, said spindle motor including:a rotor supported to be rotatable about a central axis and having an inner circumferential surface on which a magnet is provided; and a stator arranged inside the inner circumferential surface to oppose the magnet, said stator including:a stator core having an annular portion arranged to be coaxial with the central axis, a plurality of winding portions projecting from the annular portion radially with respect to the central axis, and pole portions formed on extending ends of the respective winding portions and opposing the magnet, each of the winding portions being squeezed in a direction of the central axis and having a thickness in the direction of the central axis, which is smaller than those of the annular portion and each of the pole portions, and a coil wound on each of the squeezed winding portions; said stator core having annular, plate-shaped first and second core forming members which are stacked coaxially, the first core forming members each having an annular portion, a plurality of winding forming portions extending from the annular portion radially outward, and pole portion forming portions formed at extending ends of the respective winding forming portions, each of the winding forming portions being squeezed in a direction of plate thickness and having a small thickness and a pair of bent portions which are located at two end portions of the winding forming portion in a circumferential direction of the annular portion and bent in an axial direction of the annular portion, the second core forming members each having an annular portion, a plurality of winding forming portions extending from the annular portion radially outward, and pole portion forming portions formed at extending ends of the respective winding forming portions, and the first and second core forming members being stacked such that at least one second core forming member is interposed between two first core forming members, and each winding forming portion of the second core forming member is located between two bend portions of a corresponding winding forming portion of the first core forming member. 